In many industries including onshore, offshore industries, land, sea and air transportation, a flue gas containing pollutants and greenhouse gases such as SO2, NOx and CO2 is usually generated and needs to be treated to comply with the emission standards requisitioned by various environmental authorities and international organization.
Presently, control measures for removing SO2, NOx and CO2 from the flue gas are discussed as follows.
To reduce SO2 emission, the approaches in existence are typically to use low sulfur fuel if SO2 is generated from the combustion of oils. However, the use of these approaches would be hindered due to the high cost of low sulfur fuel premium and the availability of low sulfur fuel. Lime or seawater scrubbing processes are used to reduce the SO2 emission if SO2 is generated from the combustion of coal and other combustion processes. However, the use of lime in a dry or wet environment would lead to generation of CO2, because the lime CaO is typically obtained by heating limestone CaCO3 which results in releasing CO2. That is, using the lime processes to remove SO2 is in fact an exchange for CO2 generation. In the case of using the seawater scubbing processes to remove SO2, sulfuric acid is produced due to the reaction of SO2 and water. Typically the scrubbed water has a pH ranging from 2-4, and as a result, would acidify the ocean and release CO2 from bicarbonates and carbonates commonly present in seawater. The above processes are therefore harmful to the atmosphere and marine environments, although they all are able to remove the pollutant SO2.
There have already been many low NOx engines and burners to reduce NOx emission. These low NOx combustion devices all have a common disadvantage of compromising their combustion efficiencies due to the use of a lower combustion temperature. However, the reduction in the combustion efficiency would require more fuel consumption or emit more CO2. Also, there are Selective Catalytic Reduction processes (SCR) and Selective Non-Catalytic Reduction processes (SNCR) for removing NOx in which urea or amine is used to reduce NO to nitrogen gas which is an environmental friendly gas. However, the performance of the catalysts may be affected by sulfur present in fuel oil or coal. While in scrubbing processes in which lime is used, the generation of CaSO4 would also cause inactivation of the surface of the catalysts, in turn affect the removal of NOx.
Nowadays, the processes adapted for removing CO2 are to capture CO2 by chemicals such as monoethanolamine (MEA). However, these processes suffer from a serious problem that the storage for CO2 needs to be solved. It is recognized that CO2 emission is in the order of trillion tons annually, and storing such a huge amount of the gas CO2 is costly and problematic. Also, use of chemical neutralization and absorption has been proposed from removal of CO2, but disposal of final products from these processes is a crucial issue. Therefore, they cannot be considered to be a solution against the removal of CO2.
One system and process for removing the pollutants and greenhouse gases have been proposed in the Japanese paper of Sukeon A N and Osami Nishida, titled “Electrolyzed Seawater in air pollution control of marine diesel engine” (JSME Int J Ser B (Jpn Soc Mech Eng) VOL.46; No. 1; Page 206-213 (2003)). This process is similar to the conventional processes which use alkali chemicals for scrubbing, but differs in that NaOH is produced by electrolyzing the seawater rather than added in as a ready-made NaOH chemical. However, this process is accompanied by generation of chlorine gas and HCl which are not allowed to be discharged directly by many regulations such as International Maritime Organization (IMO). In the process of this paper, the electrically produced NaOH is used to neutralize SO2, NOx and CO2, and acidic seawater of pH 3-5 is produced to oxidize NO to NO2, allowing it to react with NaOH to convert into nitrate. Again, at such low pH acidic level, carbonates and bicarbonates in the seawater would release CO2, that is, the reduction of SO2 and NO2 is an exchange of generation of CO2. In this process, CO2 is absorbed and converted into bicarbonate and carbonate in the electrolysis process. Essentially, this Japanese paper aims to remove all the three gases by converting SO2 into sulphate, converting NOx into nitrate, and converting CO2 into bicarbonate and carbonate. The products sulphate, nitrate, bicarbonate and carbonate would be discharged into the sea. Here the questions come. If nitrate produced is all discharged into the sea, the nitrate concentration would exceed the discharge limit of 60 ppm required by IMO, in light of the contents of NOx present in the flue gas. In addition, the conversion of CO2 into bicarbonate and carbonate requires the electrolysis of seawater to increase the pH of the seawater to 10, during which the content of OH ions is typically increased by about 100 ppm or 100 grams per ton of seawater. In other words, to neutralize one ton of CO2, about 386 Kg or 386,000 gm of OH ions are required in the case of a 100% neutralization of CO2. The pH 10 alkaline seawater to be produced by the electrolysis will then be 3860 tons. It would be obvious that such a process for removing CO2 is impractical due to the consumption of such high volume of alkaline water and energy as well as the huge size of an electrolytic tank.
Moreover, the pollutants and greenhouse gases SO2, NOx and CO2 are all produced and mixed together in the flue gas, and all are demanded to be removed before the flue gas is emitted. However, the above processes and systems are either intended for removing one of the pollutants and greenhouse gases, or are not a practical solution. If all these gases are to be removed together by the respective processes, installation of three different treatment plants will be required for the removal of all three gases. This inevitably results in high capital cost, substantial footprint of the three separate plants, large storage space, and high cost of reagents, in additional to the issues of storage and disposal of final products.
The processes and systems mentioned above also show that none of them is able to re-utilize CO2 or final products as an energy source and they all relate to abetment technologies but not energy recovery technologies.
The invention provides a method and a system that are capable of simultaneously removing the pollutants and greenhouse gases from a flue gas at low cost, without causing a harm to the atmosphere and marine environments and without the need of consideration of disposal of final products and storage of raw reagents. Instead, the invention utilizes the final products to recover the energy involved, which has not been taught and suggested by any of the prior art reference documents.